(a) Map of correlation delay times for the S_diff phases averaged over equal areas of 500 km × 500 km surface. An overall mean value of 3.8 s is removed. The size of dots varies with the value of the time delay. The background image displays the CMB shear velocity perturbations in the tomographic model SEISGLB2 (Durand et al. 2017). Red colour represents slower seismic velocity/positive time delay. Blue colour represents faster seismic velocity/negative time delay. (b) The size of dots represents the number of S_diff ray paths that sample each 500 km × 500 km area.